Despite the fairly extensive and universal standardization measures concerning radio telephone systems today, there are a number of different standards in use or about to be taken in use. This is also true for the digital cellular systems, out of which the following can be mentioned: JDC800 (Japan Digital Cordless; frequency range 800 MHz), JDC1500 (1500 MHz), PCN (Private Communications Network; 1500 MHz), the European GSM and the American DAMPS (Dual Mode AMPS).
For these different networks, the manufacturer makes an adaptation in connection with the manufacturing process by selecting, in a well-known way, crystal oscillators and synthesizer circuits having suitable frequencies. However, this results in overlapping and disadvantages in the coordination of the manufacturing of different mobile telephone models.
The present invention provides a circuit arrangement for a radio telephone, comprising at least one frequency synthesizer circuit (UHF1,UHF2,VHF) having an output therefrom and an oscillator (71) having a reference frequency output therefrom, the at least one synthesizer circuit (UHF1,UHF2,VHF) including a corresponding Phase Locked Loop (PLL) circuit (21,22,23) coupled to the reference frequency, and multiplier means (11,12) for frequency multiplying coupled to the output of the at least one synthesizer circuit (UHF1,UHF2,VHF) and operable by a first control signal (select X1/X2) to change the frequency of a signal output from the multiplier means (11,12), thereby selectively changing the operational frequency range of the radio telephone.
An advantage is that a radio telephone incorporating the present invention can be selectively operated in different network standards without having to have many different types of oscillator circuits. This saves on volume, weight and cost and makes such a radio telephone easier to use and more desirable to a user.